1. Field of the Invention
This invention relates to a method of removing precursors to undesirable color forming impurities prior to production of polytertramethylene ether glycol (PTMEG) and related copolymers, more specifically, but not by way of limitation, this invention relates to an improved method of removing precursors to color forming acetals in 1,4-butanediol or mixture of 1,4-butanediol with lower alkyl-substituted 1,4-butanediols by distillation prior to the ring closing cyclization to corresponding tetrahydrofuran (THF) and 3-alkyl tetrahydrofuran (3-alkyl THF) and subsequent polymerization to produce PTMEG.
2. Description of the Related Art
It is generally known and an accepted commercial practice to manufacture PTMEG from 1,4-butanediol by first catalytically cyclizing to THF, isolating the THF and then catalytically polymerizing. It is also generally acknowledged that even purified 1,4-butanediol distillate will contain small amounts of other glycols, such as 2-methyl-1,4-butanediol as well as undersirable and difficult to separate impurities, which may lead to color control problems in the final polymer and its subsequent use. For example, in U.S. Pat. No. 5,112,943 a method of preparing a high purity polymer or copolymer by cationic polymerization of THF and, if desired, an alkylene oxide, is described in which the polymer is treated with hydrogen in the present of a hydrogenation catalyst in order to reduce the color index. This same patent indicates that technical grade THF contains small amounts of impurities in a concentration of from 10 to 500 ppm, the chemical nature of which is not known in any great detail and which cause discoloration and modify reactivity in the preparation of polyesters or polyurethanes.
Even before the above suggested hydrogenation of PTMEG polymer, it was also known that certain color forming impurities in the THF could be advantageously hydrogenated prior to polymerization. For example, in U.S. Pat. No. 4,257,961 the presence of methacrolein, dihydrofurans (2,3- and 2,5-), propionaldehyde and butyraldehydes (normal and isomeric) impurities whose presence case color formation in polymeric glycols made from THF are significantly reduced by catalytic hydrogenating dry THF.
Notwithstanding the above methods of improving color in PTMEG polymer and consistent with the admission as to lack of understanding of the chemical nature of the impurities found at the ppm level, there has been a long felt need for both better understanding and control of color formation associated with the polymerization of THF to PTMEG particularly in regards to achieving production of commercial scale quantities of polymer that exhibit virtually a zero APHA color index.